Solid polysulfide rubber decaborane high energy fuel and method of preparation



United States Patent Ont-ice 3,223,563 Patented Dec. 14, 15965 3,223,563 SQLTD POLYSULFTDE RUBBER DECABURANE HIGH ENERGY FUEL AND METHOD OF PREP- ARATTUN Robert William Atteberry, Kenmore, Edward Harry Keefer, Buffalo, and William Leonard Wachtel, Niagara Falls, N.Y., assignors to (thin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed July 18, 1958, Ser. No. 749,560 8 Claims. (Cl. 149-49) This invention relates to reaction products of polysulfide type rubbers and boranes.

Polysulfide type synthetic rubbers are well known and generally prepared by the reaction of sodium polysulfide, e.g., sodium tetrasulfide, with an organic dichloride, e.g., ethylene dichloride. A well known rubber of this type is known commercially as Thiokol. Thiokols of various types are commercially available and are described more particularly below. For example, Thiokols designated LP, e.g., LP-2, LP-3 and LP-S, are prepared from sodium polysulfide and a mixture of di-2-chloroethyl formal and 1,2,3-trichloroprop'ane in the molar ratio of 98 :2 and are liquid polymers.

It has now been found that certain bo-ranes can be combined with the polysulfide type synthetic rubbers, particularly those of the LP variety, to form a solid material of high boron content suitable for use as a propellant fuel for rocket power plants and other jet propelled devices.

The fuels of this invention, when incorporated with suitable oxidizers such as ammonium perchlorate, potassium perchlorate, sodium perchlorate, ammonium nitrate, etc., burn with high flame speeds, have high heats of combustion and are of the high specific impulse type. Probably the single most import-ant factor in determining the performance of a propellant charge is the specific impulse. Appreciable increases in performance will result from the use of higher specific impulse materials. The fuels of this invention when incorporated with oxidizers are capable of being formed into a wide variety of tablets, rings and shapes with all desirable mechanical and chemical properties. Propellants produced by the methods described in this application, burn uniformly without disintegration when ignited by conventional means, such as a pyrotechnic type igniter, and are mechanically strong enough to withstand ordinary handling.

An important advantage of the fuels of this invention is that they are elastic and hence resist cracking by heat and thermal shock. for jet propelled devices may be exposed to a variety of temperature conditions. Wide variations in temperatures cause thermal expansions which may induce cracking in the propellant grains. Similarly, physical shock experienced by the propellant grains both before and during their discharge in jet propelled devices may cause fracture and cracking of the grains. Such cracked propellant grains can be extremely dangerous to the jet propelled device in that they permit the grain to burn more rapidly in the immediate vicinity of the cracked section. The uneven burning rate results may be so great as to cause an uneven thrust by the jet engine or an explosion in the rocket power plant. It is, therefore, very essential that the propellant used not be subject to fracture. Compounds having a rubber-like quality are particularly desirable for use in propellant grains because they can absorb both thermal and mechanical shock without cracking or fracturing. The natural resiliency and elastic qualities of rubher-like materials make them ideal for resisting fracture. The fuel of this invention will not deteriorate when stored over a wide range of temperature conditions. It can be produced in a uniform composition, can easily be cast into In practical use, solid propellants grains and will not shrink. In addition, the high percentage of boron which is present in the propellant composition is in a readily burnable form and results in the release of much larger quantities of energy than the corresponding composition of carbon. The fuels of this in- I vention need not use the conventional binding agent used by many other similar compounds to cause the oxidizer and fuel to adhere. The cohesive and tacky quality of the final product is sufficient to bind into a solid mass the granular oxidizing agent which is admixed with the fuel. Such mass will retain its form and elastic qualities when shaped into various configurations.

According to this invention, a polysulfide type synthetic rubber is reacted with a suitable borane to produce solid materials useful as propellant fuels.

The polysulfide type rubbers suitable for use in this invention are, as noted above, those prepared by the reaction of sodium polysulfide with an organic dichloride, i.e.,

The rubbers have the general formula (RS Many dihalides can be used, e.g., ethylene dichloride, propylene dichloride, bis(2-chloroethyl)ether, di-Z-chloroethyl for mal and 1,3-glycerol dichlorohydrin. Such products are well known and particularly known as Thiokols, a trade name of the Thiokol Chemical Corporation. They are described in the literature, for example, in Synthetic Rubber, G. S. Whitby, John Wiley & Sons, Inc, N.Y., 1954, pages 892-900, and the Encylopedia of Chemical Technology, Interscience Encylopedia, Incorporated, 1953, vol. 11, pp. 842-843. The polymers useful in this invention have an average molecular weight ranging from about 500 to about 100,000.

The average structure of Thiokols (liquid polysulfide polymers) is Thiokol LP-2 is one of the liquid polysulfide polymers manufactured by the Thiokol Chemical Corpo-ration. Chemically, Thiokol LP-2 is a polymer of bis(ethylene oxy) methane containing disulfide linkages. The polymer segments are terminated with a reactive thiol (-SH) groups; side thiol groups occur occasionally in the chain. The average structure of Thiokol LP-2 can be represented as follows, although some of the polymer segments are branched:

Similarly, Thiokol LP-3 is a liquid polysulfide polymer of low molecular weight. Unconverted Thiokol LP-3 is essentially a difuctional mercaptan made from 98 percent bis(2-chloroethyl) formal and 2 mole percent of trichloropropane, a cross-linking agent. The polymer segments are composed of a number of formal groups linked by sulfur bonds and are terminated by mercaptan groups; side mercaptan groups occur occasionally inv the chain of repeating formal units and some chain segments are crosslinked at various points. The average structure of Thiokol LP-3 can be represented as follows:

Thiokol LP-32 is one of the liquid polysulfide polymers similar to Thiokol LP-2 except that it is not crosslinked to the same extent. Converted Thiokol LP-32 has almost doubled the elongation of LP'-2, a lower modulus and a higher tear index. Thiokol LP-32 is prepared from 99.5 mole percent of bis(Z-chloroethyl) formal and 0.5 mole percent of trichloropropane. structure may be represented as follows:

The polymer is terminated by thiol (--SH) groups which also occur occasionally as side groups. The average of about 2.1 sulfur atoms link each formal group. The physical properties of Thiokols LP-2, -3, and 32 are as follows:

Its average THE PHYSICAL PROPERTIES OF THIOKOLS LP-Z LP-S LP-32 Physical State Mobile Liquid Mobile Liquid Mobile Liquid. Color Light Amber Clear Amben Clear Amber. Specific Grav' 1.27 percent--. 1.27 percent (/4 0.). Viscosity C 350-450 poises 350-450 poises. Average Molecular Weight 000 ,000. Pour Point 45 F. Flash Point (open cup) 455 F. Fire Point (open cup) 485 F. pH (water extract). 4 6.0-8.0. Moisture Content 0.1 percent (maxlmum)... 0.1-0.2.

The boranes suitable for use in this invention are decaborane and lower alkyl decarboranes in which the alkyl group contains from 1 to 5 carbon atoms. Such alkyl decaboranes include, for example, monomethyldecaborane, dimethyldecarborane, monoethyldecaborane, diethyldecaborane, triethyldeoaborane and monoisopropyl decarboane. Mixtures of lower alkyl decaboranes can be used. Alkyl decaboranes can be prepared by reacting decaborane and an alkyl halide in the presence of an alkylat-ion catalyst as described in pending application Serial No. 497,407, now Patent No. 2,999,117, filed March 28, 1955, of Altwicker et al. and in pending application Serial No. 540,141, now Patent No. 3,109,030, filed October 12, 1955, of Altwicker et al. wherein ferric chloride is used as the catalyst. For example, the alkylated deoab-oranes can be obtained from a Friedel-Crafts reaction of decaborane-14 (B H and an ethylhalide, in the presence of a ferric chloride catalyst. The product of this reaction consists of a mixture of monoethyldecaborane, diethyldecaborane and triethyldecaborane. These constituents can be separated by a fractional distillation at reduced pressures. At a pressure of about 0.2 mm. of mercury monoethyldecaborane will boil at approximately C., diethyldecaborane at C. and triethyldecaborane at 65 C. By this method of separation products are obtained having a purity of greater than 99 percent. Monoethyldecaborane can be also prepared by the reaction of decaborane and ethylene as described in pending application Serial No. 514,121, filed June 8, 1955, of Stange et al. Also alkylated decaboranes such as monoethyldecaborane and diethyldecaborane can be prepared by reacting a monolefin hydrocarbon with a mixture of decaborane and an aluminum halide as described in pending application Serial No. 557,634, filed January 6, 1956, of Neil et al., now Patent No. 2,987,552.

The polysulfide type synthetic rubbers are generally reacted with the borane at elevated temperatures and at atmospheric pressure although elevated pressure can be used, e.g., from 1 to 10 atmospheres. The reaction temperatues can be varied widely, e.g., between 135 C., but preferably between and 90 C. The quantities of the reactants can also be varied between Wide limits, such as 0.01 to 6 moles of the borane to one mole (based on average weight of the repeating unit) of the polysulfide type synthetic rubber. Best results however seem to be obtained when about 2 moles of the borane are reacted with one mole of the polysulfide type rubber. Expressed in weight ratios the ratios of the reactants can vary from about .005 to 10.0, and preferably about 0.74 to 2.21, parts of the borane per part of the polysulfide rubber. The reaction generally is complete in about 0.5 to 10 hours, depending on the ratio of reactants and the temperature and pressure employed. The reaction can be The solid materials produced by the reaction described above are elastic or rubber-like solids of varying hardness and colors. Some are tacky. They are non-pyrophoric but burn readily on ignition. The solids contain substantial proportions of boron and the boron is linked to the rubber molecule, probably between two or more molecules.

The invention will be further illustrated by the following examples in which the term moles is indicative of gram moles.

Example I 8.9 grams of Tln'okol LP-32 (a liquid polysulfide polymer prepared from 99.5 percent mole of bis(2-chloroethyl) formal and 0.5 percent mole of trichloropropane) and 13.4 grams of decaborane were mixed intimately while under a nitrogen atmosphere and placed in a 2- necked 50 ml. round bottom flask. A gas burette leading to a conventional gas flow meter was connected to one neck of this round bottom flask and an inlet second neck through which nitrogen was introduced before the reaction. The flask was placed in an oil bath :at a temperature of C. and left for 195 minutes, during which time a gas, later analyzed as hydrogen, was evolved at the rate of approximately 2.2 cc. per minute. The oil bath was then heated to C. for 95 minutes during which time hydrogen evolved at 5 cc. per minute. Then the flask was removed, allowed to cool and a yellowish-orange resilient solid material found therein removed. Infrared analysis of this solid revealed boron hydrogen bonds and decaborane. Wet chemical analysis of the material indicated the following:

Percent Boron 52.39

Carbon 11.45

Hydrogen 7.20 Sulfur 13.5

Infrared analysis indicated a boron-Thiokol linkage. The solid was ground in a mortar, then placed in a Soxhlet extractor and extracted fro three hours using normal pentane as a solvent (cycle 5 to 7 minutes).

This type of extractor boils the solvent in a lower flask and allows the solvent vapor to pass into a condensor mounted above. Here the vapor condenses and drips upon a thimble-like vessel containing the material to be extracted. This thimble is positioned in a reservoir containing a syphon device which, when the solvent reaches a level suflicient to cover the extracted material, returns the solvent and solute dissolved from the extracted material to the lower flask. The extractor operates continuously and automatically when the lower flask is heated, dripping pure solvent on the extracted material and retaining the solute-rich solvent in the lower flask. The Wet chemical analysis was as follows:

Percent Boron 45.31 Carbon 16.73 Hydrogen 7.42 Sulfur 14.90

This solid proved to be non-pyrophoric but burned vigorously with a green flame when ignited.

Example II 11.94 grams of Thiokol (LP-2) [a polymer prepared from di(chloroethyl) formal (98 percent) and trichloropropane (2 percent) and containing disulfide linkages] was placed in a 230 ml. glass flask and 17.59 grams of decaborane added under an atmosphere of nitrogen. The flask was then placed in an oil bath and the temperature raised to 72 C. over a period of one hour. The flask was held at 72 for an additional hour and a half then allowed to cool to room temperature. Gas, later analyzed as hydrogen, evolving from the reaction was collected in a glass burette and measured to be 345 cc. It was then observed that the flask contained a solid rubber-like material. The solid matter was placed in a Soxhlet extractor with 200 ml. of pentane and extracted once every 15 minutes for a period of five hours. The solid material was then removed from the thimble in the extractor and the solvent which was on the surface Was removed therefrom by exposing to a vacuum of 10* mm. of mercury for about 12 hours. The solvent used in the Soxhlet extractor was then analyzed and found to contain decaborane. Solid material from the Soxlet extractor was removed from the vacuum and worked by hand to uncover any possible inclusions. It was then washed in methylcyclohexane for an hour and a half and dried again under vacuum. The resulting solid was a yellowish-green tacky elastomer which was not pyrophoric. On ignition it burned readily with a green flame. Infrared analysis indicated boron hydrogen bonds. Chemical analysis of this solid was as follows:

Percent Boron 24.7 Carbon 21.7 Hydrogen 5.8 Sulfur 29.3 Oxygen 1 14.5

Total 96.0

Calculation based on the weight ratio of elements in the starting materials.

Example HI 12.16 grams (73.1 miilimoles) of Thiokol LP-2 (a polymer of bis(ethylene oxy) methane containing disulfide linkages) was mixed in a 230 cc. flask under a nitrogen atmosphere with 18.02 grams (147.1 millimoles) of decaborane. No solvent was used. The flask was placed in an oil bath and raised to a temperature of 75 C. in /3 of an hour. The flash was then heated from 75-80 for an additional 2 /3 hours. During this time, 1370 cc. of a gas, later analyzed as hydrogen, were collected. The solid formed in the flask was then placed in a Soxhlet-type extractor and cycled under a nitrogen atmosphere for about 12 hours using 200 ml. of 2,3-dimethylbutane (cycling period 10-15 minutes). The extracted solid mate rial was then placed under a vacuum (10- mm. of mercury) and then evaporated to dryness for several hours. It was then Worked by spatula, cup up, washed with 2,3- dimethylbutane and again dried under vacuum. The solid produced was a white tacky non-pyrophoric elastomer which on ignition burned vigorously with a green flame.

Infrared analysis indicated a compound decaborane. The wet analysis was:

of Thiokol and Calculation based on the weight ratio of elements in the starting materials.

Example IV 12.59 grams (75.5 millirnoles) of Thiokol LP3 (a difunctional mercaptan made from 98 mole percent of bis(2 chloroethyl) formal and 2 mole percent trichloropropane) and 18.71 grams (153.0 millimoles) of decaborane were mixed thoroughly in a 200 ml. glass flask under a nitrogen atmosphere. The flask was then placed in an oil bath and the temperature raised to 7580 C. and maintained at that temperature for two hours. 1660 cc. (74.1 millimoles) of gas later analyzed as hydrogen evolved. The flask was. then cooled and the resulting solid removed and placed in a Soxhlet-type extractor (cycle time about 20 minutes) where it was extracted under a nitrogen atmosphere for 8 /2 hours using 200 ml. of 2,3-dimethylbutane (previously dried over calcium sulfate). The extracted solid was dried under a vacuum (10* mm. of mercury) for several hours. The solid Was then removed and appeared to be a tacky, yellow, rubberlike material which upon analysis yielded the following results:

Percent Boron 48.2 Sulfur 18.3 Carbon 17.2 Hydrogen 9.2 Oxygen 9.1

Total 102 Calculation based on the weight ratio of elements in the starting materials.

The boron containing solid materials of this invention can be employed as ingredients of solid propellant compositions in accordance with general procedures which are Well understood in the art, since the solid materials are readily oxidized using conventional solid oxidizers, such as ammonium perchlorate, potassium perchlorate, sodium perchlorate, aluminum perchlorate, lithium perchlorate, ammonium nitrate and the like. In formulating a solid propellant composition employing one of the boron containing solid materials of this invention, generally from 10 to 35 parts by weight of the boron containing solid material and from 65 to parts by weight of oxidizer such as ammonium perchlorate are present in the final propellant composition.

In the propellant, the oxidizer and the boron containing solid material are formulated in intimate admixture with each other, as by finally subdividing each of the materials separately and thereafter intimately admixing them. The purpose in doing this, as the art is aware, is to provide a proper burning characteristic in the final propellant. The subdividing of the borane containing solid reaction product can be accomplished by means of conventional equipment designed to reduce the size of resilient materials, e.g. rubber, such as devices which cut, chop or tear the feed material into a divided product, e.g., the wellknown rotary knife cutters and granulators. The oxidizer can be reduced in size by conventional equipment, e.g., a hammer mill. The admixing of the finely divided reaction product and finely divided oxidizer can be accomplished by conventional mixing equipment employing rotating blades, e.g., kneaders or Baker-Perkins mixers.

In addition to the oxidizer and oxidizable material, the final propellant can contain an artificial binder. A1-

though, in general, it is not necessary to use such a binder with the fuels of this invention, one can be employed to secure additional binding strength, or where a propellant fuel of different physical characteristics is desired. A binder of an artificial resin type, generally containing ureaformaldehyde or phenol-formaldehyde, is particularly useful. The function of this resin is to give the propellant mechanical strength and at the same time improve its burning characteristics. In manufacturing a suitable propellant using a binder, proper proportions of finely divided oxidizer and finely divided boron-containing material are admixed With a high solids content solution of a partially condensed urea-formaldehyde or phenol-formaldehyde resin, the proportion being such that the amount of the resin is about from to 10 percent by weight, based upon the weight of the oxidizer and the boron compound. The ingredients are thoroughly mixed with the simultaneous removal of the solvent, e.g., by use of a vacuum, and following this the solvent-free mixture is molded into the desired shape as by extrusion. Thereafter, the resin in the mixture can be cured, e.g., by heating at moderate temperatures, e.g. 25100 C. Conventional and suitable methods of formulation of solvent propellant compositions are further described in US. Patent 2,622,277 of Bonnell and U.S. Patent 2,646,596 of Thomas.

When the final propellant is to be made without hinder or binding agent, the technique is very similar. In such a case, the oxidizer and the solid reaction product of this invention are again formulated in intimate admixture with each other, as by subdividing each of the materials separately and then intimately admixing them. The result of this admixture is a homogeneous solid having strong cohesive qualities, which will retain its form well, and which can be formed into various shapes as by extrusion. The molding can be done in conventional equipment designed for this purpose such as hydraulic extruders. The extruded propellant can then be cut into pieces and trimmed as desired.

The following examples illustrate the formulation of solid propellant compositions utilizing the solid boron containing reaction product of this invention.

Example V The solid boron containing material of Example I in an amount of 10 pounds is finely divided to a size of about 100 microns in a rotary knife cutter. Ammonium perchlorate (30 pounds) oxidizer is finely divided to a size of about 20 microns in a hammer mill. The two finely divided materials are then introduced into a rotating-blade type mixer and intimately admixed. The resulting homogeneous cohesive solid is molded and extruded and cut into cylindrical grains of about 3 inches in diameter and 10 inches in length. The resulting solid grains are an i11- timate admixture of solid fuel and oxidizer.

Example VI The solid boron containing material of Example II and ammonium perchlorate oxidizer in the amounts of Example V are finely divided as in Example V and are introduced into a rotating-blade type mixer along with one pound of urea-formaldehyde resin in chloropropylene oxide and the three components intimately admixed with the simultaneous removal of the solvent by means of a vacuum. The resulting solvent-free homogeneous solid is molded and extruded to form grains as in Example V. The grains are then heated in an oven to about 100 C. for 2 hours to cure the resin in the grains. The resulting grains are an intimate admixture of solid fuel and oxidizer with resin binder.

The product of this invention can be utilized in its extracted form, that is, after the uncombined decaborane or lower alkyl decaborane has been removed from the final reaction product by extraction with a suitable solvent. The unextracted reaction product, however, can also be advantageously used as a rocket propellant. The

unextracted product contains a quantity of decaborane or lower alkyl decaborane homogeneously dispersed throughout. When combined with an oxidizing agent in the manner previously described and ignited, both the combined product and the decaborane or lower alkyl decaborane burn readily. The unextracted product has the advantage of containing a higher percentage of boron (an element of high heat of combustion) homogeneously dispersed in a form which can be easily admixed with a suitable oxidizing agent and which will burn liberating a larger quantity of energy than the extracted product.

What is claimed is:

1. A solid high energy fuel composition consisting essentially of the product of reaction of a polysulfide rubber and a borane selected from the group consisting of decaborane and lower alkyl decaboranes at a temperature of about 50 to 135 C. and in amounts of about 0.005 to 10.0 parts by weight of borane per part of rubber.

2. The composition of claim 1 in which the rubber and borane are reacted in amounts of about 0.74 to 2.21 parts by weight of borane per part of rubber at a temperature of about 65 to C.

3. The composition of claim 1 in which the polysulfide rubber is a liquid polysulfide polymer prepared from sodium polysulfide and a mixture of bis(Z-chloroethyl) formal and trichloropropane and the borane is decaborane.

4. The composition of claim 3 in which the rubber and decaborane are reacted in amounts of about 0.74 to 2.21 parts of decaborane per part of rubber at a temperature of about 65 to 90 C.

5. The method of preparing a solid high energy fuel composition which comprises reacting a polysulfide rubber with a borane selected from the group consisting of decaborane and lower alkyl decaboranes at a temperature of about 50 to 135 C. and in amounts of about 0.005 to parts by weight of borane per part of rubber.

6. The method of claim 5 in which the rubber and borane are reacted in amounts of about 0.74 to 2.21 parts by weight of borane per part of rubber and at a temperature of about 65 to 90 C.

7. The method of claim 5 in which the polysulfide rubber is prepared from a mixture of bis(2-chloroethyl) formal and trichloropropane and the borane is decaborane.

8. The method of claim 7 in which the polysulfide rubher is prepared from a mixture of bis(Z-chloroethyl) formal and trichloropropane and the borane is decaborane.

References Cited by the Examiner UNITED STATES PATENTS 6/1951 Hurd et al. 5/1957 Moore et al.

OTHER REFERENCES Jet Propulsion, Air Technical Service Command (1946), p. 158. Ley, Coast Artillery Journal, January- February 1948, p. 27.

Hurd, Chemistry of the Hydrides, New York, John Wiley & Sons, Inc/(1952), p. 94.

Arendale, Industrial and Engineering Chemistry, vol. 48, No. 4, April 1956, pp. 725-6.

Chem. and Eng. News, May 27, 1957, pp. 18-23.

Ritchey, Chem. and Eng. News, Nov. 11, 1957, pp. 78-82. Chem. and Eng. News, Jan. 20, 1958, pp. 2829.

The Wall Street Journal, vol. CLI, Feb. 13, 1958, No. 31, p. 1, col. 1 and p. 21, cols. 2-4.

Major, Chemical Engineering Progress, vol. 54, No. 3, March 1958, pp. 49-54.

Proell et al., The Journal of Space Flight, vol. 2, No. 1, pp. 1-9.

CARL D. QUARFORTH, Primary Examiner.

ROGER L. CAMPBELL, LEON D. ROSDOL,

Examiners. 

1. A SOLID HIGH ENERGY FUEL COMPOSITION CONSISTING ESSENTIALLY OF THE PRODUCT OF REACTION OF A POLYSULFIDE RUBBER AND A BORANE SELECTED FROM THE GROUP CONSITING OF DECABORANE AND LOWER ALKYL DECABORANES AT A TEMPERATURE OF ABOUT 50 TO 135*C. AND IN AMOUNTS OF ABOUT 0.005 TO 10.0 PARTS BY WEIGHT OF BORANE PER PART OF RUBBER. 